(1) Field of the Invention
This invention relates to radiation detectors of the direct conversion type including an amorphous selenium semiconductor film (hereinafter referred to as “a-Se semiconductor film” where appropriate) sensitive to radiation, for use in the medical, industrial, nuclear and other fields.
(2) Description of the Related Art
An indirect conversion type radiation detector first converts radiation (e.g. X rays) into light, and then converts the light into electric signals by photoelectric conversion. As distinct from the indirect conversion type, a direct conversion type radiation detector converts incident radiation (e.g. X rays) directly into electric signals (charges) by an a-Se semiconductor film sensitive to radiation. FIG. 1 is a schematic sectional view showing a basic construction of a conventional radiation detector of the direct conversion type.
The radiation detector shown in FIG. 1 includes an active matrix substrate 51, a radiation sensitive a-Se semiconductor film 52, and a common electrode 53 for bias voltage application. Numerous collecting electrodes (not shown) are formed on the surface of the active matrix substrate 51, in a two-dimensional matrix arrangement set within a radiation detection effective area SA. An electric circuit (not shown) is arranged on the surface of the active matrix substrate 51 for storing and reading electric charges collected by the respective collecting electrodes upon incidence of radiation. The radiation sensitive a-Se semiconductor film 52 is laid on the surface of the active matrix substrate 51 where the collecting electrodes are formed, to generate charges upon incidence of the radiation. The common electrode 53 for bias voltage application is formed two-dimensionally on the front surface of the a-Se semiconductor film 52.
In time of radiation detection by the conventional radiation detector, a bias voltage is applied from a bias voltage source to the common electrode 53. With the bias voltage applied, electric charges are generated in the radiation sensitive a-Se semiconductor film 52 upon incidence of the radiation. The electric charges generated in the a-Se semiconductor film 52 are collected by the collecting electrodes. The electric charges collected by the collecting electrodes are fetched as radiation detection signals from the respective collecting electrodes by the storing and reading electric circuit including capacitors, switching elements and electric wires.
That is, in the direct conversion type radiation detector shown in FIG. 1, each of the collecting electrodes in the two-dimensional matrix arrangement acts as an electrode corresponding to each pixel in a radiographic image (pixel electrode). Radiation detection signals obtained can be used to create a radiographic image according to a two-dimensional intensity distribution of the radiation projected to the radiation detection effective area SA.
The radiation sensitive a-Se semiconductor film 52 having a large area may be formed by PVD (Physical vapor deposition), typically, for example, vacuum deposition. Thus, the radiation detector can easily secure a large radiation detection effective area SA. In the case of the conventional radiation detector, as shown in Japanese Unexamined Patent Publication No. 2001-26443, the common electrode 53 for bias voltage application formed on the a-Se semiconductor film 52 comprises an aluminum or MgAg film of about 100 nanometers (nm) (=1,000 angstroms).
However, the conventional radiation detector with the above construction has a drawback that defects could occur with the radiation sensitive a-Se semiconductor film 52 when the a-Se semiconductor film 52 is damaged by the heat of vapor deposition for forming the common electrode 53 for bias voltage application on the a-Se semiconductor film 52. The a-Se semiconductor film 52 with many defects lacks in long-term reliability, and detection errors caused by the defects result in low-quality radiographic images.
Further, the conventional radiation detector has a drawback of being low in bonding strength between the radiation sensitive a-Se semiconductor film 52 and common electrode 53 for bias voltage application. Where an aluminum or MgAg film of about 100 nanometers (nm) (=1,000 angstroms) is used, the common electrode 53 has a weak bonding strength, and hence low reliability.